Organic electroluminescence (“EL”) display devices of a type comprising a matrix of a large number of self-luminous organic EL elements hold great promise as the next generation of image display devices since they require no back-lighting nor do they restrict viewing angles.
The organic EL elements are light-emitting elements of a current-driven type, of which brightness can be controlled by an amount of electric current flowed through them. There are simple matrix type and active matrix type as the methods of driving the organic EL elements. The former has a drawback that it is difficult to produce a large-scale and high-definition display although it only needs simple pixel circuits. It is for this reason that the efforts are being made actively in recent years for development of organic EL display devices of the active matrix type, which is composed of a matrix of pixel circuits having organic EL elements, each provided with a driver transistor for driving the current-driven type light-emitting element.
The driver transistor and the peripheral circuit are formed generally of thin film transistors. There are thin film transistors of a type made of polysilicon and another type made of amorphous silicon. The amorphous silicon thin-film transistors are suitable for large-scale organic EL display devices since they feature a high uniformity in mobility, easy to fit for upsizing, and inexpensive, although they have some weaknesses such as poor mobility and large changes in the threshold voltage with time. There have been some studies conducted for measures to overcome the weakness, or the changes in the threshold voltage with time, of the amorphous silicon thin-film transistors by improvements of the pixel circuits. Patent document 1, for instance, discloses an organic EL display device having pixel circuits capable of displaying a stable image by keeping an amount of the currents supplied to the light-emitting elements free from influence of the threshold voltage of thin film transistors even when the threshold voltage changes.
According to the pixel circuits disclosed in the patent document 1, however, it is necessary to pulse-driving a common line in connection with cathodes of the plurality of organic EL elements. Since the plurality of organic EL elements has a large electrostatic capacitive component, the common line momentarily draws a large current when it is pulse-driven. It thus has a problem that the circuit for driving the common line bears a large load, and therefore not suitable for a large-scale image display device.
Furthermore, the pixel circuit described in the patent document 1 is a drive circuit designed on a condition that it uses enhancement-type transistors with a positive threshold voltage as the driver transistors. Therefore, it does not allow use of depletion-type transistors with a negative threshold voltage as the driver transistors. It is desirable, however, that the pixel circuits are operable with any of the enhancement-type transistors and the depletion-type transistors in order to increase the flexibility of manufacturing the thin-film transistors and to deal with the changes in the threshold voltage with time.
In addition, it is necessary to compose the image circuits only with N-channel transistors since the N-channel transistors are the only type that is now in practical use as amorphous-silicon thin-film transistors for large-scale image display devices. Moreover, it is also preferable that the circuits have a structure allowing connections of anodes of organic EL elements to sources of the driver transistors and cathodes of the organic EL elements to a common electrode so as to ease the manufacturing of the organic EL elements.    [Patent Document 1] Japanese Patent Unexamined Publication, No. 2004-295131